This invention relates generally to pressure control apparatus and methods. The invention relates more particularly, but not by way of limitation, to an apparatus and a method for controlling an autoclave valve, a tank valve and a piston of a pump within a fluid circuit for pressurizing an autoclave. The invention also relates more particularly, but not by way of limitation, to an apparatus and method for controlling pressure in a plurality of autoclaves, wherein each autoclave is connected to a respective one of a plurality of valves of a fluid circuit pressurized by a single pump.
In the oil and gas industry, cement slurries are pumped into well bores for various reasons, one of which is for cementing casing. Different well bores can have different temperature and pressure conditions which can affect different cement slurries; therefore, it is desirable to be able to test a particular cement slurry for its suitability for a particular well bore environment. Such testing is well known in the industry.
A typical cement test occurs in a closed container, such as an autoclave, wherein the contents can be maintained under pressure. In at least some applications, heat can also be applied to heat the contents to a desired temperature. The pressure and temperature, when controlled, are typically those anticipated to be encountered downhole. Specific examples of such test equipment are the Halliburton Services Cement Consistometer and the Halliburton Services Ultra-sonic Cement Analyzer (UCA). See also U.S. Pat. No. 4,622,846 to Moon, Jr. et al. and U.S. Pat. No. 4,653,313 to Sabins et al.
To obtain a good test, the pressure (and temperature, if controlled) should be maintained at the desired level. It has been observed, however, that once pressure has been set in an autoclave, for example, the pressure can change over time due to leaks in the system or expansion of the cement under test, for example. There is the need, therefore, for a pressure control apparatus and method by which a desired pressure can be automatically maintained in an autoclave (or other pressure vessel) once it has been established.
In conducting tests of the aforementioned type, it is also sometimes desirable to test multiple samples at the same time. Thus, samples of the same batch of cement could be subjected to different pressures (and temperatures, if desired) to determine how the cement would react to different conditions which might be met in the well bore. Such multiple tests could be run by individual autoclaves and pressurizing equipment; however, it would be more efficient if a number of autoclaves could be pressurized, either to one or more constant levels or in accordance with one or more varying pressure schedules, by a single pressurizing system. That is, there is the need for apparatus and method by which multiple pressure vessels can be multiplexed to a single pressurizing system.